Energy 134 (2017) 649e658 Contents lists available at ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy The economics of electricity generation from Gulf Stream currents * Binghui Li a, , Anderson Rodrigo de Queiroz a, Joseph F. DeCarolis a, John Bane b, Ruoying He c, Andrew G. Keeler d, Vincent S. Neary e a Department of Civil, Construction, & Environmental Engineering, North Carolina State University, Raleigh, NC, 27695-7908, United States b Department of Marine Sciences, University of North Carolina, Chapel Hill, NC, 27599-3300, United States c Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, NC, 27695-7908, United States d UNC Coastal Studies Institute, Wanchese, NC, 27981, United States e Water Power Technologies Department, Sandia National Laboratory, Albuquerque, NM, 87123, United States article info abstract Article history: Hydrokinetic turbines harnessing energy from ocean currents represent a potential low carbon electricity Received 6 January 2017 source. This study provides a detailed techno-economic assessment of ocean turbines operating in the Received in revised form Gulf Stream off the North Carolina coast. Using hindcast data from a high-resolution ocean circulation 27 May 2017 model in conjunction with the US Department of Energy's reference model 4 (RM4) for ocean turbines, Accepted 9 June 2017 we examine resource quality and apply portfolio optimization to identify the best candidate sites for Available online 9 June 2017 ocean turbine deployment. We find that the lowest average levelized cost of electricity (LCOE) from a single site can reach 400 $/MWh. By optimally selecting geographically dispersed sites and taking Keywords: Ocean current energy advantage of economies of scale, the variations in total energy output can be reduced by an order of Gulf Stream magnitude while keeping the LCOE below 300 $/MWh. Power take-off and transmission infrastructure Portfolio optimization are the largest cost drivers, and variation in resource quality can have a significant influence on the Energy economics project LCOE. While this study focuses on a limited spatial domain, it provides a framework to assess the Renewable generation techno-economic feasibility of ocean current energy in other western boundary currents. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction electricity generation, especially the Gulf Stream [5e7,10e12], the Kuroshio Current [13,14], and the Agulhas Current [15e17]. These Marine energy resources, which include ocean waves, tides, are all jet-like oceanic western boundary currents, which are open ocean currents as well as gradients in ocean temperature [1] among the swiftest large-scale marine currents. Their current and salinity [2], could serve as an important low carbon renewable speeds are fast enough to be considered excellent energy resources energy source. Previous research has shown great potential for [11,18]. These western boundary currents typically are thousands of marine electricity generation worldwide [3e9]. The available ki- km in length, about 100 km wide, extend to at least 1000 m depth, netic energy in US coastal waters associated with wave, tidal, and and have the strongest current speed at the surface near the center ocean current energy resources is estimated to be 1170 TWh/yr [8], of the current [11]. 222e334 TWh/yr [9] and 45e163 TWhyr [7], respectively. As the most intensely studied ocean current, the Gulf Stream Ocean currents (i.e., non-tidal marine currents) are seawater begins in the Caribbean and terminates in the North Atlantic Ocean. circulations driven by a combination of wind, density, and pressure This fast moving ocean current brings a significant amount of heat differences in the ocean [1]. Ocean currents mostly flow horizon- and salt to the European continent, and also provides an opportu- tally and typically have their highest flow velocities near the sur- nity for energy capture. In the US, the most plausible locations to face. On average, they will have a prevailing direction, but temporal harness Gulf Stream energy are in the Florida Straits and off the variability can at times be strong. Ocean currents have been studied North Carolina coast, the two locations where the current makes its over the past several decades as a potential energy source for closest approach to shore. The estimated extractable energy from the Florida Current (i.e., the portion of the Gulf Stream within the Florida Straits) ranges from 1 GW to 10 GW [5e7], and the portion of the Gulf Stream within 200 miles of the US coastline between * Corresponding author. Florida and North Carolina can yield approximately 9 GW or E-mail address: [email protected] (B. Li). http://dx.doi.org/10.1016/j.energy.2017.06.048 0360-5442/© 2017 Elsevier Ltd. All rights reserved. 650 B. Li et al. / Energy 134 (2017) 649e658 80 TWh per year of electrical power [7]. tracer (temperature and salinity) advections were solved with a Most ocean currents exhibit some degree of path meandering third-order upstream scheme in the horizontal direction and a [19e25] as well as periods of acceleration and deceleration. As a fourth-order centered scheme in the vertical direction. The hori- consequence, the ocean current velocity at a specific location is zontal mixing for both the momentum and tracer utilized the subject to temporal variability [26]. Previous studies [19,23,24,27] harmonic formulation with 100 and 20 m2/s as the momentum and have shown that the lateral movements of the Gulf Stream from tracer mixing coefficient, respectively. Turbulent mixing for both the Florida Straits to Cape Hatteras, NC can be significant due to momentum and tracers was computed using the Mellor/Yamada wind forcing, flow instabilities, and bathymetric effects. Along the Level-2.5 closure scheme [39]. For open boundary conditions, the southeastern US coast, the standard deviation of the lateral Gulf model was nested inside the 1/12 global data assimilative HYCOM/ Stream position displacement increases from 5 to 10 km within the NCODA [40] output superimposed with the 6 major tidal constit- Florida Straits to an approximate 40 km local maximum down- uent forcing derived from an ADCIRC tidal model [41] simulation of stream of a bottom topographic feature off Charleston, SC known as the western Atlantic. the "Charleston Bump" (31e32 N latitude). The standard devia- The MABSAB sub-domain selected for analysis was 77 Wto74 tion in the Stream's lateral displacement decreases moving north- W, 33 Nto36 N, which includes the strongest, near-shore Gulf eastward (downstream) from the Bump, to approximately Stream current off the North Carolina coast. While the fastest Gulf 10e20 km at Cape Hatteras, NC. These path variations will influence Stream currents are closest to the surface, we assume the turbines the cost-effectiveness of Gulf Stream energy extraction. are installed at a depth of approximately 50 m below the sea sur- Previous work includes technical assessments of marine turbine face to accommodate the drafts of large ships and to keep turbine design and performance, mostly to address tidal energy applica- hardware out of the surface wave zone. The study domain is shown tions [28e35]. In addition, a detailed cost analysis for a hypothetical in Fig. 1: the 334 km  280 km rectangle is discretized into a project in the Florida Straits has been performed [10,36]. However, 2km 2 km mesh grid with 19,188 grid points. Daily average the Florida Current is confined within the Florida Straits, which is current speeds for the years 2009e2014 are used in this study approximately 100 km wide between the Florida peninsula and the [38,42,43]. Bahama Banks. By contrast, significant meanders are observed for The electricity output of the turbine at a given current velocity is the Gulf Stream off the North Carolina coast [11,19]. While resource expressed by the following equation: assessments conducted at several discrete locations near Cape Hatteras, NC indicate potential for commercial development 1 PðvÞ¼ hC rAv3 (1) [11,37], they do not tie ocean current resource estimates to the 2 p economic performance of turbine arrays. This paper represents a significant extension of existing work by where v is the current velocity, A is the swept rotor area, r is the providing a comprehensive techno-economic assessment of ocean density of sea water, Cp is the power coefficient that accounts for current energy off the North Carolina coast. Our analysis is the first the conversion of current power to mechanical power, and h is the to combine a multi-year resource assessment based on output from combined power chain conversion efficiency, which includes the a high-resolution ocean circulation model, a portfolio optimization gearbox, generator, transformer and power inverter efficiencies to identify optimal locations to install turbine arrays, and a lev- (see Supplementary Table F for values of the parameters). The elized cost analysis that considers the tradeoff between resource design and performance of the ocean current turbine is adopted quality and distance to shore. Furthermore, this paper represents from Neary et al. [10]. The design represents a moored glider with the first application of portfolio optimization in order to identify a four axial flow marine turbines. The rated capacity of each turbine diverse set of generation sites that hedge against the risk of future is 1 MW, and the total capacity of each unit is 4 MW. The power Gulf Stream meanders. The structure is as follows. Section 2 de- curve is adapted from Neary et al. [10] but adjusted for the lower scribes the model assumptions used for resource assessment and the techno-economic study, and introduces the portfolio optimi- zation model. The results are presented in Section 3, and Section 4 describes the insights and conclusions from this study. 2. Methods 2.1. Resource characterization Gulf Stream resource data is obtained from a realistic high- resolution regional ocean circulation model, which is used to hindcast the circulation of the Middle Atlantic Bight (MAB), South Atlantic Bight (SAB) and parts of Gulf Stream, Slope Sea, and Sar- gasso Sea [38].